Following on from replacing the radiator fan switch, I next turned my attention to the thermostat.
The thermostat regulates the water temperature in the engine. When the engine is cold, the thermostat blocks cooled water from entering the engine and just allows the water to circulate around the engine - allowing it to warm up quicker.
As soon as the water warms up (usually around 88 degrees Celsius), the thermostat opens, and allows cooled water to flow from the radiator into the engine. A faulty thermostat will mean the water runs hot if it's stuck closed, or cold if stuck open.
So here is how I changed it on my VW T4 2.4d.
The first thing to do is put the van on some ramps - if you haven't got these, then jack up the front of the van and secure on axle stands. The way I did it was by putting the van on the driveway, and positioning the front end so it overhung the kerb. I then chocked the wheels so it couldn't move. This allowed enough access to get under the front of the engine as shown in the photo below.
Removing the belly pan
Before the job can start in earnest, it is necessary to remove the belly pan under the engine. This is pretty straight forward, with 13mm bolts to be undone around the edge of the pan, and two 10mm nuts to removed towards the rear of the pan.
Take it easy as the pan can be heavy if it is the metal type (mine was). I managed to remove it by myself, by using a jack to support it as I undid the bolts. I then lowered it on the jack and removed it from under the van.
Removing the thermostat housing
To remove it, you undo the two 10mm bolts and pull the housing away. Now at this point, all the water in the cooling system is going to come gushing out, so make sure you have a suitable container to catch the water. Yes, you're going to get a bit wet.
An alternative way is to take the lower hose off the radiator and drain the water out first. This will involve putting the radiator into the service position, and removing some of the front end panels - so if you're just changing the thermostat, its quicker to just remove the thermostat housing and catch the water from there.
Replacing the thermostat
The photo on the right, shows the old thermostat removed from the van, and the correct location for the rubber O-ring. The "spring" end of the thermostat fits into the engine block.
Optional, but recommended: Before replacing your thermostat, you can now flush your engine with clean water. Unscrew the blue cap on your header tank at the top of the engine, (if you haven't already done so!) and hold a hose pipe in the tank. This will force water around the engine water ways, and then drain out the thermostat hole. Flush it through until clean water comes out of the thermostat hole.
I got my new thermostat from GSF, and it cost about £15, and came with a new rubber O-ring. Its worth checking a new thermostat by placing it in a pan of boiling water to make sure it opens correctly. Mine did open fine when the pan started to simmer - which is about right, 88 degrees Celsius.
I fitted the new thermostat into the block, and then placed the O-ring into position. The housing can then be bolted back into position. The housing should bite down onto the O ring to make a water tight seal, and simultaneously secure the thermostat into position.
Replacing the coolant
With the thermostat housing back in position, your engine coolant needs to be replaced. Volkswagen recommend what is known as G12 coolant/antifreeze - it can be identified by its red/pink colour. I bought 2 litres and poured it straight into the header tank, and then topped it up with water. This gave me about a 33% antifreeze mix.
I filled up the system to the "max" line in the header tank. This is because as soon as you start the engine, the water level will drop as it fills the engine water ways and the radiator. Now have a helper start the engine - as soon as the engine starts you will notice the water level drop - keep pouring water into the head tank until it shows signs of stabilising around the "min" line.
Bleeding the system
With the coolant stabilising around the "min" line, its now time to get your assistant to blip the throttle pedal a few times, this will force any air locks to bubble up into the header tank. Keep an eye on the level, and add more water as necessary, to keep the level to around the "min" line or just above.
Heater bleeding: Now get your assistant to turn the heater on, and make sure it is set to hot. In the engine bay, the heater pipe has a bleed screw at the top of the system (see pic on right). Undo this until coolant runs out, and all air has been expelled. It helps to have the throttle blipped a couple of times with the screw undone. Once all the air has been expelled, nip up the bleed screw.
Run the van up to temperature, blipping the throttle every now and again. Check the radiator top hose gets warm/hot, and the lower radiator hose is cooler. Top up the water as necessary as air gets expelled into the header tank. My water level settled just above the "min" line which is about where it should be.
Checking for leaks
With the engine running, check for leaks around the thermostat housing. If there is, it could mean the housing bolts aren't tight enough, or the O-ring seal hasn't seated properly, and would need adjusting. Luckily for me, it didn't show any signs of leaking.
To make sure, I left the belly pan off the van for a few days whilst running it around town. I checked each morning for pools of coolant under the van - there was none. So confident I didn't have any leaks, I could finish the job by replacing the belly pan under the engine - which is the reverse of taking it off!
So has it worked - yes, to a degree! The temperature is a lot more stable on the dashboard, but the dash is still reading a bit out - so again, its looking like the common inaccurate dash problem.
The only way to know for sure will be to add an after-market temp gauge and sender - which is probably what I will do shortly. In the meantime, I know the thermostat and the radiator fan switch are working correctly - so at least I have peace of mind.